feat: add data-driven skin/theme engine for CLI customization

Adds a skin system that lets users customize the CLI's visual appearance
through data files (YAML) rather than code changes. Skins define: color
palette, spinner faces/verbs/wings, branding text, and tool output prefix.

New files:
- hermes_cli/skin_engine.py — SkinConfig dataclass, built-in skins
  (default, ares, mono, slate), YAML loader for user skins from
  ~/.hermes/skins/, skin management API
- tests/hermes_cli/test_skin_engine.py — 26 tests covering config,
  built-in skins, user YAML skins, display integration

Modified files:
- agent/display.py — skin-aware spinner wings, faces, verbs, tool prefix
- hermes_cli/banner.py — skin-aware banner colors (title, border, accent,
  dim, text, session) via _skin_color()/_skin_branding() helpers
- cli.py — /skin command handler, skin init from config, skin-aware
  response box label and welcome message
- hermes_cli/config.py — add display.skin default
- hermes_cli/commands.py — add /skin to slash commands

Built-in skins:
- default: classic Hermes gold/kawaii
- ares: crimson/bronze war-god theme (from community PRs #579/#725)
- mono: clean grayscale
- slate: cool blue developer theme

User skins: drop a YAML file in ~/.hermes/skins/ with name, colors,
spinner, branding, and tool_prefix fields. Missing values inherit from
the default skin.

hermes_cli/config.py

@@ -89,6 +89,14 @@ DEFAULT_CONFIG = {
         "record_sessions": False,  # Auto-record browser sessions as WebM videos
     },
     
+    # Filesystem checkpoints — automatic snapshots before destructive file ops.
+    # When enabled, the agent takes a snapshot of the working directory once per
+    # conversation turn (on first write_file/patch call).  Use /rollback to restore.
+    "checkpoints": {
+        "enabled": False,
+        "max_snapshots": 50,  # Max checkpoints to keep per directory
+    },
+    
     "compression": {
         "enabled": True,
         "threshold": 0.85,
@@ -112,8 +120,9 @@ DEFAULT_CONFIG = {
     "display": {
         "compact": False,
         "personality": "kawaii",
-        "resume_display": "full",  # "full" (show previous messages) | "minimal" (one-liner only)
-        "bell_on_complete": False,  # Play terminal bell (\a) when agent finishes a response
+        "resume_display": "full",
+        "bell_on_complete": False,
+        "skin": "default",
     },
     
     # Text-to-speech configuration
@@ -171,7 +180,7 @@ DEFAULT_CONFIG = {
     "command_allowlist": [],
 
     # Config schema version - bump this when adding new required fields
-    "_config_version": 5,
+    "_config_version": 6,
 }
 
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